Hmmm. According to Mr G. that should give you 60-odd dB of attenuation if the construction is tight, but I'm loathe to discard James' advice about outside wall mass - he does know what he's talking about. Is adding another layer of drywall to the outside structure outside of budget?

Given your overall budget, and the risk of not getting what you need, is there a local acoustician (I don't think that's a real word) you can get to review the project? It will be an additional expense but it might be worth it.

I'm not convinced the Rockwool in the void is doing anything useful, it simply doesn't have enough mass (and this is after filling the voids between my studio ceiling joists with rock wool 20 years ago). Mass for lows, airtightness for highs and you should be good to go. I'd probably spend more money on extra layers of plasterboard (with green glue) on either or both of the inner or outer walls and maximum effort to ensure airtightness when the doors are shut. A small crack os poorly designed baffles in the ventilation system will defeat 50% of your attenuation a stroke.

I'm pretty sure green glue isn't a actually a glue as it never really sets. It's viscous and will help grab things, but by itself I believe it's not going to hold weatherboard in place. When I helped Max put up a lot of the plasterboard in his last home studio, I know the plasterboard was held in place with screws as well as having a layer of green glue between each layer..

The joints between the weatherboards are also going to have to be acoustically sealed. Even small cracks can really let a lot of sound out. It's all about the fine detail.

I know I may be labouring the point but I'm trying to help you avoid a major mistake. It really would be worth finding out how much more expensive a blockwork outer later would be. I think you are in danger of tying yourself in knots with ever more exotic materials when the simple solution is best. I really cannot see how a wood based outer skin is going to do much unless you leave a very large air gap of at least 0.5m between outer and inner layers.

You are also missing a trick on the inner layer - instead of two layers of the same thickness of plasterboard it is usually recommended to use different thicknesses - Gyproc suggest using 19mm plasterboard plank next to the joists followed by a layer of 12mm acoustic plasterboard for a sound attenuating wall. This can be enhanced by a further later of 15mm acoustic plasterboard with Green Glue between each layer.

I'd also agree with Sam that the insulation batts between the layers are superfluous (although the insulation between studs is useful).

jmch wrote:If I did switch to a block exterior wall I guess the interior framing and insulation would be used still. What would the roof have to be made of? Thanks

Yes - you'd still use the interior framing. My roof is tiled with a layer of 15mm acoustic plasterboard under the rafters and a gap of around 200mm between plasterboard and the internal structure below.

One other tip - I've used carpet underlay made from recycled car tyres to decouple the inner structure from the concrete base.

blinddrew wrote:That's the STC rating, so should be a measure from 125Hz to 4kHz.

The attenuation at 125Hz can be much less than 60dB in a structure with an STC rating of 60 - in fact, if I understand the calculation correctly, it could be as low as 36dB and still be called STC 60 if the higher frequencies are sufficiently attenuated. Lower frequencies are ignored by the STC rating.

This image also shows how much more effective blocks are at low frequencies (the block wall has a rating of 47 while the stud wall has a rating of 48).

I've bitten the bullet and asked my builder to switch to a concrete block exterior. James I took your point about the block foundations already being there..

The exterior walls will now be block.. the interior decoupled room within a room a timber frame with double layer acoustic board as before (walls and ceiling)

But we're mid job now and I don't know a couple of things.. yet

What will the exterior roof need to be? My budget definitely won't stretch to a concrete roof. James, if I tile the exterior roof and put acoustic plaster board under the rafters, with a gap before the interior double board ceiling:- will that be enough mass to match the block walls?- won't there be another air gap between the board under the rafters and the tiled roof (the 3 leaf set up?)

Also, on the exterior block walls, my builder is talking about putting batons on the outside of the block, and putting the timber cladding on them. Will the gap between the block and the cladding matter? Should I insulate that?

jmch wrote:What will the exterior roof need to be? My budget definitely won't stretch to a concrete roof. James, if I tile the exterior roof and put acoustic plaster board under the rafters, with a gap before the interior double board ceiling:- will that be enough mass to match the block walls?- won't there be another air gap between the board under the rafters and the tiled roof (the 3 leaf set up?)

That roof construction is exactly what I have here - using fairly substantial tiles. It won't quite match the block walls but on mine there's a bigger air gap between the layers which helps. The larger air gap also reduces the effect of the 3 leaves and, most importantly in my case, allows for thick roof joists to ensure that the inner ceiling wouldn't bend too much if I found that I had to use 3 layers of heavy plasterboard.

jmch wrote:Also, on the exterior block walls, my builder is talking about putting batons on the outside of the block, and putting the timber cladding on them. Will the gap between the block and the cladding matter? Should I insulate that?

My biggest worry would be the outer cladding resonating with the vibrations coming through the wall and therefore radiating more noise to the outside world than the wall alone. I'd suggest some kind of damping material behind the cladding although I'm not sure exactly what material to suggest as you also need air circulation behind the cladding to prevent damp patches.

James Perrett wrote:My biggest worry would be the outer cladding resonating with the vibrations coming through the wall and therefore radiating more noise to the outside world than the wall alone. I'd suggest some kind of damping material behind the cladding although I'm not sure exactly what material to suggest as you also need air circulation behind the cladding to prevent damp patches.

I'd have thought that as long as your batons were reasonably frequently spaced then any resonant frequencies wouldn't be too low down the frequency range and so should be ameliorated by what's inside of them?Alternatively, how about a strip of rockwool, parallel and between each baton just to damp any potential resonance? Wouldn't need to be thick or particularly wide I wouldn't have thought?

good grief leave you lot alone for 5 minutes and you all grab wandering minstrels and send up them up several different garden paths.... (all bar James )

there are many ways to skin the cat, very few of them leave you without claw marks, and a criminal record , and old ladies haranguing you in the post office.

(or so i believe )

useful materials for roof.

Cement Bonded Particle board. vastly higher density and mass than any other board product , a bastard to work with, but if you can't do a block and beam roof, then this stuff is your best bet.... can be generally handled and worked like Ply/OSB but will generate a lot more dust, and wear out blades, and need more bodies to move it.... it's effectively chipboard, made with cement rather than glue

as an inner layer above insulation , below felt and tiles, it is very useful for adding a dense inert mass layer.

you can use steel or timber framing for roof, Steel uses less depth, and copes better with the load and is more stable across the seasons.

Green glue IS NOT A GLUE . you CANNOT bond anything to anything else with it, expect possibly your hair to your teeshirt...

it does not act as a shock absorber in a straight line through the material sandwich, it acts laterally ... energy transmission in multilayer wall structures occurs largely by flexing of the plane of the material sheet, and the sheets move over each other in flexing, in a lateral domain, green glue acts as a dampener to this action, reducing it's magnitude and time scale, effectively making the board sandwich act more like one solid layer, but with an element of self dampening, much like a limp high density rubber ... thus resonance times are shorter, and transmission levels are lower.